1. Field of the Invention
The present invention relates to a heat treatment system, and more particularly, to a pressure control in a processing furnace.
2. Description of the Related Art
A method of manufacturing a semiconductor device includes, as an example of a heat treatment, an oxidation treatment by which an oxide film is formed on a surface of a semiconductor wafer. In one example of the oxidation treatment, a semiconductor wafer is brought into contact with water vapor at a predetermined temperature in a processing furnace so as to oxidize the semiconductor wafer (wet oxidation). A system for wet oxidation treatment is disclosed in Japanese Patent Laid-Open Publication No. JP 63-210501 A. The system includes a combustion unit arranged outside a processing furnace to generate water vapor by reacting hydrogen gas with oxygen gas. The water vapor thus generated by the combustion unit is supplied to the processing furnace to perform a heat treatment.
Such a heat treatment system may have an exhaust system which is connected to a factory exhaust system of slightly reduced pressure. In order to evacuate a processing furnace to a predetermined pressure, the heat treatment apparatus includes the exhaust system provided with an exhaust pressure control valve of a butterfly type or an exhaust pressure control valve whose opening is adjusted by a stepping motor and a spring. Exhaust pressure in the exhaust system is controlled by adjusting the opening of the control valve based on pressure measured by a differential manometer arranged in the exhaust system.
In the event that the exhaust pressure control valve is of a butterfly type, since a water film is formed between the valve element and a pipe because of condensation of water vapor, control of the opening of the valve tends to be unstable. In order to prevent this, an air-inlet port must be arranged upstream or downstream of the valve. In the event that the exhaust pressure control valve is of a type whose opening is adjusted by a stepping motor and a spring, an inert gas must be introduced to the valve for achieving smooth valve operation and a stable pressure control, which complicates the valve structure.
In the event that the opening of the valve is controlled based only on pressure measured by the differential manometer, when atmospheric pressure changes due to change of the weather, pressure in a processing furnace also varies. This results in variation in the oxide film thickness. In recent years, there has been required to form a quite thin film, and the tolerance range of the film thickness has been strictly limited. Thus, variation in film thickness, which was negligible in the past, becomes a critical problem.
Moreover, the differential manometer is intentionally configured so that a zero-point adjustment is impossible, because it is used for measuring a slight differential pressure. Thus, when the differential manometer is used for a long time, it is difficult to maintain its measuring accuracy.